Because they designed these six person encounters to be played within certain parameters and when people find ways to exploit them it's a problem. Getting the Mythoclast and other HM loot by throwing grenades at Atheon, for instance, was a big problem.
Also, I imagine it's a bit easier to do things like modify death zones or create invisible walls than it is to alter an ordering algorithm that effects your character, gear and every subsequent item property in the game.
With that said, I don't understand why they don't just disable the heavy ammo perk for now like they did with Hive Disruptor---maybe it's not that simple.
